In recent year, a solar cell (solar cell module) has been widely employed as a device directly converting solar energy into electric energy from the viewpoints of the effective use of natural resources and prevention of environment pollution. Further development of solar cells is in progress.
As shown in FIG. 1, a solar cell generally includes a transparent front side protection material 11 (e.g., glass plate), a front side sealing film 13A, plural photovoltaic elements 14 (e.g., photovoltaic elements made of silicon), a backside sealing film 13B and a backside protection material 12 (backside covering member). For preparing a solar cell, first, the foregoing members are successively laminated in this order. Then, the laminated body is subjected to vacuum degassing, subsequently, heating under pressure for croslinking or curing a front side sealing film 13A and a backside sealing film 13B to adhesively combine the laminated body.
In order to generate a large electrical output, a solar cell has plural photovoltaic elements 14 connected with each other. Therefore, sealing films 13A, 13B having high insulation properties seal the photovoltaic elements for ensuring electrical insulation properties between the photovoltaic elements 14.
Furthermore, developments of thin-film solar cells, such as thin-film silicon type solar cell, an amorphous silicon film type solar cell and copper indium selenide (CIS) type solar cell are also developed. These thin-film solar cells are prepared by, for example, forming a thin-film photovoltaic element such as a semiconductor layer on surface of a transparent material such as a glass substrate and a polyimide substrate by chemical vapor deposition method or the like, superposing a sealing film on the thin-film photovoltaic element and adhesively combining them.
As a sealing film for the solar cells, a film made of ethylene-polar monomer copolymer such as ethylene-vinyl acetate copolymer (occasionally abbreviated to EVA) or ethylene-ethylacrylate copolymer (EEA) is used. Especially, the ethylene-vinyl acetate copolymer is preferably used because it has low cost and high transparency. Furthermore, in order to ensure mechanical durability of the solar cell, and prevent internal electrical leads and electrodes from rust caused by humidity or permeated water, it is required that an ethylene-polar monomer copolymer film used in sealing films is combined with a glass plate, photovoltaic elements and a backside covering member by high adhesive strength.
For this reason, in the conventional solar cell sealing film, the addition of organic peroxides as crosslinker to an ethylene-polar monomer copolymer gives crosslinked structure to improve weather resistance, and furthermore, the addition of silane-coupling agents to an ethylene-polar monomer copolymer brings about the improvement of adhesive strength (Patent Document 1).
Recently, a solar cell having a longer operating life has been required. Further, increasing use of a solar cell under unforgiving circumstances such as desert region or tropical area requires that a solar cell sealing film also has more enhanced durability and weather resistance, especially durability that maintains adhesive performance for a long duration under the circumstances of a high temperature and high humidity.